In the preparation of an ethylene-.alpha.-olefin copolymer in a hydrocarbon solvent in the presence of a catalyst comprising a vanadium compound and an organoaluminum compound, or a catalyst comprising a vanadium compound, an organoaluminum compound, and a halogenated ester compound, if the catalytic ingredients, etc. remain or are incorporated in the copolymer as a final product, problems such as discoloration of the copolymer and deterioration in weather resistance and heat resistance of the copolymer occur. Hence, the catalytic ingredients, etc. remained in the copolymer obtained by the polymerization reaction must be completely removed from the copolymer.
Conventional methods for removing the catalytic ingredients from the ethylene-.alpha.-olefin copolymer prepared in the presence of a vanadium-based catalyst include a method wherein a large amount of water is added to a polymer solution, the mixture is mixed in a mixer, the catalyst is extracted into an aqueous phase, the aqueous phase is separated from an organic phase, and the polymer containing a reduced amount of the catalytic ingredients is recovered from the organic phase; and a method wherein water or an alcohol is added to a slurry obtained by a copolymerization reaction in a solvent which does not dissolve therein the polymer, to reduce the amount of catalytic residue in the polymer [see, Yasuharu Saeki, Polymer Production Process, published by Kogyo Chosakai (1971)].
However, these methods have some disadvantages described below.
In the method wherein a polymer solution is treated with a large amount of water, it is considered that the catalytic ingredients which are solubilized in water by bringing them into contact with water are extracted with water. However, when one expects that the catalytic ingredients are more completely removed, the mixing of the polymer solution with water must be thoroughly made, and a large-size, heavy-duty mixer must be used at a high speed (at high revolution number). Hence, much costs of equipment and power are required and, therefore, the method is not industrially advantageous.
In the method using an alcohol, etc., the alcohol must be purified and reused and, hence, not only the process is complicated, but an energy consumption is increased. Thus, the method is not industrially advantageous, too.
As other methods, JP-A-63-275605 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method wherein copolymers prepared in the presence of a titanium-based catalyst are washed with water containing an alkaline compound; and JP-B-43-6471 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses a method wherein copolymers prepared in the presence of a vanadium-based catalyst are oxidized and then washed with water containing an alkaline compound.
In recent years, since ethylene-.alpha.-olefin rubbery copolymers are mixed with other resins such as polypropylene and widely used in the fields of automobile parts and packaging materials, the requirements on weather resistance, non-discoloration, etc. of the rubbery copolymers have become severer. In order to meet the requirements, it is highly demanded to further reduce the amount of the catalytic ingredients remained in the rubbery copolymer. For example, it is demanded that the amount of the vanadium compound remained in the copolymer is reduced to not higher than 1 ppm in terms of V.sub.2 O.sub.5.
Further, there is a problem that in addition to the metallic catalytic ingredients, halogenated ester compounds used as activating agents for the polymerization reaction are remained in the copolymer. When the halogenated ester compounds are remained in the copolymer, such a problem as lowering in the quality of the copolymer caused by halogens occurs, that is, corrosion of processing machines and coloration of the copolymers occur so that the copolymers are no longer applicable to vessels and packaging materials for foods and medicines.
Under these circumstances, the conventional techniques described in the aforesaid patent specifications are not yet satisfactory.
Thus, in order to solve such problems as mentioned above, the present inventors have made extensive researches and eventually accomplished the present invention.